It's also worth mentioning that Pentax produced a 43mm prime, because it is the perfect "normal" lense by the aforementioned standards. Why they insist on other weird focal lengths like 31mm & 77mm no one knows (witchcraft perhaps?).
P.S. Sorry for the OT.
On topic:
1. Wireless Flash:
The advantages of Nikon's flash/lighting system should not be underestimated if you have an interest in flash photography. It completely changes the way you work with flashes relative to the Canon system (or the Pentax, Olympus, & Sony systems for that matter).
It's phenomenal. Then again in my experience, the vast majority of amateur/hobbiest photographers don't use flash at all or only in the most simplistic contexts for which Nikon's system is complete overkill.
2. TTL & e-TTL:
On that note, everyone has TTL, through-the-lens metering for their flashes now. But I believe that only Canon & Nikon can use the focus distance from the AF to judge power without firing a test shot (which causes people to blink as well as lots of annoying missed shots when people think the first flash indicates the exposure when it doesn't). This is very nice if you use a flash on auto. OTOH I've personally have never used a flash on auto... Flash is a beast for which the "correct exposure" in the computer's mind is so often different from the correct exposure aesthetically as to render the computer's opinion completely useless so often that it's
never worth bothering with.
3. Availability of IS
Bear in mind that Olympus, Pentax, & Sony all have in-body IS in at least some of their bodies. This is a substantial advantage that should be considered. IS combined with a fast prime can be remarkably useful in some situations. It gives you IS with a
vast collection of 1st-party & 3rd-party lenses. I would not have bought the excellently-reviewed Sigma 100-300mm f/4 if I was not using a Pentax body with built-in IS. Because I am however, this lense becomes the best choice for me (I'd take it over Nikon or Canon's great 70-200mm's). Recently Canon has put IS into some of its shorter zooms to make up for the lack of in-body IS, but nothing in primes.
4. Lense Selection:
Nikon & Canon have a very complete selection of zooms, but neither is interested in producing high-quality, updated primes. Pentax on the other hand is very interested in providing a wide variety of high-quality, compact prime lenses, but is currently lacking a selection of 1st-party telephoto zooms. This will be somewhat remedied by the 50-135mm f/2.8 & 60-250mm f/4 (which is a phenomenally useful focal length range on cropped sensors). I say "somewhat remedied" because they're both DA* lenses (L in Canon speak) and are expensive, and not small. Olympus has probably the highest quality, most awesome zooming glass available, but only one prime (a sin considering what they could do with some compact primes and that 4/3 sensor!).
A little more on Pentax because I moved to them from Nikon recently and feel the need to comment in a little more depth on what I have learned through the transition. Pentax has a tendency to flesh out the niches in its lense line-up, creating things that no one else has. They have 3, super-compact, pancake primes (
pic of the super-compactness ) for example. They have a 10-17mm fisheye zoom, which is actually shockingly useful & fun. A lightweight 16-45mm f/4 zoom (a very useful focal length in cropped sensor format) in addition to the faster (but bigger & heavier) 16-50mm f/2.8 which they offer against the 17-55mm f/2.8's from Nikon & Canon. The difference between the FOV of a 16mm & a 17mm is much more significant than it sounds, but Pentax felt like the range would be more useful on wide end than on the long. They made the effort to hit the classic 24mm focal length at the wide end. (And I completely agree that it was worth it: 50-55 is a desert of opportunities --pull out a portrait lense or just shoot normal, but 16 over 17 is handy all the time). Was it more difficult? Probably. Does 16-45 sound remotely like a conventional focal length? Not at all, but that's Pentax for better or worse.
So, they really go the extra mile to flesh out the sytem and offer unique glass that you can't get elsewhere; they don't stick to the hot sellers like Canon & Nikon. (For example, is the optically uncompromising, built like a tank 31mm f/1.4 a profit maker maker? Primes don't sell much in this day and age. I'm sure they make a little on it, but why spend resources developing that when there are surer bets? Who knows but Pentax does. Perhaps they see it as the one way to stay alive against Canon & Nikon.)
5. Body Ergonomics:
Now, bodies even among manufacturers differ a great deal, but there are some generalizations that most detached people would make. Nikon control layouts have always just made sense. Nikon cares about this a great deal. Canon bodies, on the other hand, --not to say they're bad-- tend to have little things that vary from little layout idiosyncrasies to brain dead stupid (They include a dedicated Pictbridge "Print" button, but make it impossible to even customize a mirror lock-up button on a pro body!). The company also stubbornly insists they know better. Pros have complained about the lack of mirror lock-up forever and gotten exactly nowhere.
Pentax, again, get a little unconventional with the ergonomics. In Programmed Auto, you can switch into Shutter Priority or Aperture Priority just by moving the appropriate control wheel. Hitting the magic green button (near the shutter release) sends you back to Programmed Auto. This is the kind of simple, special touch that makes shooting with the camera easier/better, and it makes so much sense it now infuriates me that I can't get this feature anywhere else. Once you get used to it, it boggles your mind that you can't adjust the shutter speed up a couple stops in low light, or stop the aperture down for a greater DOF during that quick portrait-landscape when out hiking without reaching for the mode dial on other cameras. This displeases me greatly on other bodies now... (I know it doesn't sound like much, but the little things...)
In manual, that same green button instantly dials in the Programmed Auto exposure. Simple, small touch? Yes. Useful, saves time? Again, surprisingly often... These are features that you didn't know you couldn't live without until you try them.
6 Viewfinders:
The view finder is one of the most important thing for most people, and for cropped frame bodies Pentax > Nikon > Canon. This holds true at the bottom for K100D vs D40/D50/D70/D80 vs Rebel XTi/Rebel XT, as well as in the mid-range for the K10D vs the D200 vs the Canon 30D/40D. The 40D may match the D200. Nikon's new D300 ups the ante a bit D300. It has 100% frame coverage which no one offers except in their pro bodies (Pentax doesn't have a "pro" body). The magnification is still less than the K10D, but it's a barely bigger viewfinder thanks to that 100% coverage. Obviously things are different for full-frame bodies (Word is D3 > Canon 1Ds MkII > Canon 5D, don't know about the MkIII), and none of the cropped-frame bodies compare with their full-frame competitors.
7. Weather Sealing:
Pentax has sealed the K10D. For Canon & Nikon you have to go up to the pro bodies. I think, but can't confirm, that Canon & Nikon's latest generation prosumer bodies, the D300 & 40D, may have some sort of weather resistance.
Okay, that's enough. My musings became an essay. I hope some of it is useful, and I apologize for the Pentax pimping (if it came across that way) --I'm not a fanboy, there just happens to be quite a variety of things I like about the little things they pay attention to in fleshing out the system (I didn't actually realize there where that many things until I started writing it all out).
. 
